In many code reading systems information is represented by relative characteristics of the bits, such as light and dark, black and white, magnetic and non-magnetic (less magnetic), retroreflective and non-retroreflective, red and blue, as well as by the relative size of the bits. The signal from the reader represents the contrasting characteristics by its positive and negative levels. In order to determine whether a bit is in one category or the other in such relationships, e.g. black or white, it is necessary to compare the reader signal with a reference or threshold level. If it is greater than the reference it is one characteristic, i.e. white; if less, black. However, due to variations in: circuit operation and supply; distance and orientation between the reader and code; contrast levels of the code bits; condition of the code elements, e.g. dirty, torn, obliterated; the "black" and "white" levels actually can vary greatly. Thus in some cases the reference level may be so high or so low that the different bits are not recognized. Further, in many cases, such as where the size as well as the nature of the bits is important, even small variations in the reference level relative to the reader signal level can be extremely significant: the measurement of the size of the bits always should be taken at the same relative level of the reader signal, optimally the one half amplitude point.
To attempt to overcome these problems, in some systems a variable reference level is provided by a system which tracks or follows the upper (positive) and lower (negative) levels of the signal and produces a third signal at some intermediate level, e.g. midway between. These positive and negative follower circuits typically use a storage device, e.g. an R.C. network, to store the peak positive and negative levels. The capacitor in the positive follower circuit charges to the positive peaks and discharges between them while the negative follower discharges to the negative peaks and charges between them. With no coded label present, i.e. with the reader viewing "free air", the reader signal is at either a generally positive or negative level: with an optical reader of black and white codes on a white label, where black or minimum light return to the reader is negative, the "free air" signal is negative. Thus the positive peak and negative peak signals initially follow the reference signal closely, with the negative peak signal generally at the level of the reader input signal in the "free air" condition. Subsequently, when the reader encounters the white label, the reader signal goes positive and the positive follower tracks it. However, in the meantime the negative peak signal continues to increase in the positive direction as the negative follower circuit charges. Thus, if the time constant of the negative follower circuit is perfectly suited to this system, label, and all the relevant conditions, then when the first black code is reached the negative peak signal intersects with the reader signal at the negative peak of the reader signal. But this is never the case. Either the negative follower time constant is too fast, in which case it intersects the reader signal above the negative peak and thus establishes the reference level higher, more positive, than it should be and results in a wider apparent width of the black bit; or the negative follower time constant is too slow, in which case it may not intersect the reader signal until all or a number of the bits have been read and thus establishes the reference level lower, more negative than it should be and results in a narrower apparent width of the black bit. Attempts to overcome this difficulty have resulted in systems in which the various time constants, reference levels and other critical settings have been made adjustable. In this approach, each system must be fine tuned for internal system differences and for the particular labels, environment and manner in which it is to be used. But, even then, the variations in label quality, condition and orientation, system components and ambient conditions require constant attention and returning to maintain satisfactory operation.